The overall objectives of the proposed research continue to be elucidation of the biochemical determinants of drug action and the development of synergistic drug combinations for the treatment of viral infections. The unsual interactions between 5'-aminothymidine (5'-AdThd) and viral and host cell thymidine kinases suggest several new approaches which may lead to improvements in the therapy of herpes virus infections. Firstly, elucidation of the role of feedback inhibition and possible antagonism of this effect in regulating the activation of antiherpes compounds is proposed. The ability of 5'-AdThd and other 5'-amino derivatives to disrupt the regulation of thymidine kinase activities and, thereby, to enhance the activation of antiviral nucleosides in herpes simplex virus (HSV) and varicella zoster virus (VZV) infected cells will be determined. Secondly, 5'-AdThd inhibits the phosphorylation and, therefore, the toxicity of trifluorothymidine (CF3dUrd) in unifected cells. In addition, CF3dUrd is an excellent inhibitor of human cytomegalovirus (HCMV) in vitro. Since no effective therapy for this infection in vivo is available, the ability of CF3dUrd and 5'-AdThd, in combination, to selectively inhibit HCMV replication will be assessed. The pattern of CF3dUrd metabolism alone, and in combination with 5'-AdThd, in HCMV infected cells will be elucidated. Thirdly, based on genetic data, it has been suggested that thymidine kinase is an important determinant of interferon's antiviral activity. In some cells 5'-AdThd can modulate thymidine kinase activity in a biphasic manner. At low concentrations 5'-AdThd can stimulate enzyme activity by antagonizing thymidine triphosphate mediated feedback inhibition. At higher concentrations, however, interactions with the active site of thymidine kinase predominate and activity is inhibited. This novel property suggests a biochemical approach to evaluate the influence of thymidine kinase on interferon activity. Using 5'-AdThd, the antiherpes and antiproliferative actions of interferon will be evaluated under conditions of control, stimulated and inhibited levels of thymidine kinase activity. Experiments in both thymidine kinase positive and negative cells will be conducted. This approach could provide a biochemical means of modulating interferon activity.